1. Field of the Invention
The invention relates to a wire-cut electrical discharge machine, more particularly, to a wire-cut electrical discharge machine having a wire electrode cutting device capable of cutting off a predetermined length of a wire electrode, which is broken by an accident, before rethreading the wire electrode.
2. Description of the Related Art
In the wire-cut electrical discharge machine, electrical discharge pulses are supplied in a machining zone between a wire electrode and a workpiece in order to cut a complex shape from the workpiece. In such a wire-cut electrical discharge machine, there is a need to forcibly cut off the wire electrode in two cases under ordinary working conditions. For example, after one workpiece has been cut, a new workpiece is to be set for cutting. At this time, the wire electrode must be forcibly cut off to smooth the wire electrode so that it can be placed into a starting position of the new workpiece. A similar case occurs when the wire electrode needs to be forcibly cut off because there is a pattern comprising several parts disposed independently of each other. To cut out the pattern from the workpiece, one part is first cut from the workpiece and, prior to cutting the next part from the workpiece, the wire electrode must be forcibly cut off so that the wire electrode can be placed into a starting position for that next part.
To cut the workpiece, electrical discharge pulses are supplied in the machining zone between the wire electrode and the workpiece. However, if the electrical discharge repeatedly occurs at the same point of the wire electrode, there is a probability that the wire electrode will break at that point. Upon such a accidental breakage of the wire electrode, a predetermined length of the wire electrode must be cut off before the wire electrode is rethreaded. Especially when the wire electrode is broken by the accident, such as described above, the breaking point of the wire electrode (the pointed head of the wire electrode) or the periphery thereof is usually deformed by the electrical discharges occurring in the machining zone between the wire electrode and the workpiece. Therefore, it is difficult to direct the wire electrode, whose point head is deformed and ragged, into a small wire guide of the wire-cut electrical discharge machine or into the starting position on the workpiece, that is, it is difficult to rethread the wire electrode in the wire-cut electrical discharge machine.
As a result, in the wire-cut electrical discharge machine, following a breakage, a predetermined length of the deformed wire electrode is cut off and scrapped. Then, since the wire electrode is sharp at the pointed head thereof, it is relatively simple to insert the wire electrode into the wire guide and to rethread the wire electrode in the wire-cut electrical discharge machine. Several types of wire electrode cutting devices capable of automatically executing the above-mentioned operations have been disclosed.
As a type of wire electrode cutting device, there is the wire electrode cutting device disclosed in U.S. Pat. No. 4,698,478. According to the disclosure of that wire electrode cutting device, the wire electrode to be cut off is tensioned between a brake device and a clamp device. The brake device is used for applying a back tension to the wire electrode upstream in the feeding direction of the wire electrode for machining. The rolling-type clamp device is disposed above the workpiece. The tensioned wire electrode is cut off by a cutting device disposed above the clamp device, and the bundle of scrap wire around the clamp is pushed off so as to fall in a disposal chute.
According to the above-mentioned structure, the pointed head of the broken-down wire electrode is wound around the clamp device so as to apply the tension. Therefore, after the wire electrode is cut off, the scrap wire electrode is tightly wound around the clamp device so that it is very difficult to push the bundle of scrap wire off the clamp device. As a result, the scrap wire push-off disposal device becomes very complex.
A wire-cut electrical discharge machine capable of solving the above-mentioned problems is disclosed in Japanese Laid-Open Patent Publication No. 2-237724. In the disclosed wire-cut electrical discharge machine, a supply bobbin for holding the wire electrode, a brake for applying a braking force to the wire electrode, a cutting device having a pair of welding electrodes, a pair of tension rollers, a disposal box beside the tension rollers, and a detection circuit for detecting a breakage of the wire electrode are provided.
The wire electrode is fed from the supply bobbin across the brake and around a direction change pulley to a point where it is held by a pair of clamp rollers immediately above the cutting device. Immediately below the cutting device are the pair of tension rollers between which the wire electrode is passed. One of the pair of tension rollers is driven by a motor in a feeding direction for feeding the wire electrode. Provided immediately to the side of the pair of tension rollers is the disposal box.
When the wire electrode is to be cut, the brake is applied and the driven tension roller continues to feed the wire electrode thereby applying a tension to the wire electrode between the brake and the tension rollers. With the tension applied, the wire electrode is cut by the welding electrodes. At that time, the motor driving the driven tension roller must be reversed to feed the cut off broken portion of the wire electrode in a direction opposite to the feed direction so that it may be received in the disposal box. Thus, the timing for reversal of the motor driving the driven tension roller is critical, if the separation between the welding electrodes and the tension rollers is comparatively small, or the cut off broken portion of the wire electrode will be fed into the machining zone, and released from the tension rollers, before the reversal of the motor and the driven tension roller.
On the other hand, should the distance between the welding electrodes and the pair of tension rollers be large, so as to reduce the timing criticality, the machine will be unnecessarily and undesirably enlarged. In either case, the need to reverse the motor and the driven tension roller so as to dispose of the cut off broken portion of the wire electrode increases the complexity and the cost of the apparatus.